Added beginnings of shader composition with the shells of the ShaderComposer and ShaderAttribute classes. This aren't

functional yet, but a useful placeholders for future fucntionality.

include/osg/StateAttribute

@@ -35,6 +35,7 @@ namespace osg {
 // forward declare NodeVisitor, State & StateSet
 class NodeVisitor;
 class State;
+class ShaderComposer;
 class StateSet;
 class Texture;
 
@@ -315,6 +316,9 @@ class OSG_EXPORT StateAttribute : public Object
           */
         virtual void apply(State&) const {}
 
+        /* compose associated shaders via the ShaderComposer. */
+        virtual void compose(ShaderComposer& composer) const {}
+
         /** Default to nothing to compile - all state is applied immediately. */
         virtual void compileGLObjects(State&) const {}